How to disable SharePoint subsite creation option for owners

Microsoft SharePoint users can organize their data using Site Collections and Sub sites. A Site Collection can have multiple sub sites underneath it, with same or different permissions.

While sub sites are a good way to organize sub sections or groups within a top level site, creating many can lead to a mess with unmanaged data and permissions. Microsoft suggests using site collections with Hub sites instead of multiple sub sites.

In that case, we may want to restrict sub site creation option for site owners.
You can turn off subsite creation option from SharePoint Admin Center using below steps.

1. Login to the SharePoint Online Admin Center https://code2care-admin.sharepoint.com.
2. Click 'Settings' > scroll down and click 'Classic settings page'.
3. Under section 'Subsite Creation' > select 'Disable subsite creation for all sites' > Click OK.

Read the description carefully

Control subsite creation for people who have permission to create sites.
This controls visibility of the Subsite option on the Site contents page and enables new subsite creation.
Use hub sites to connect related sites instead of using subsites.

And the 3 available options,

Disable subsite creation for all sites - disabled fully
Enable sub site creation for classic sites only - enabled only for classic sites
Enable subsite creation for all sites - enabled fully

4. Wait .. wait .. wait. It may take few minutes for the changes to take effect, typically 30 minutes.
5. Navigate back to your site and try creating a sub site.
6. Now users no longer see the option 'subsite' in the dropdown while creating new content.

For SharePoint on-premises, navigate using these steps for the option

Manage Web Applications > Web App > User Permission > Create Subsites > uncheck > Save

❓️ What if a smart user tries to access the subsite creation link directly https://code2care.sharepoint.com/_layouts/15/newsbweb.aspx ?

You are sorted, they will see an error message "New subsites are not available for your organization. Create a new site instead."